Because there are many different types of communication systems, such as GSM, DCS, PCS, DAMPS, and others, it is increasingly possible to have different types of systems serving a common area. These systems typically operate at different frequency ranges, e.g., GSM typically operates at 890-960 MHz and DCS typically operates at 1710-1880 MHz. In the future, it may be desirable to introduce any of a number of functions such as home based wireless phone, mini data link, wireless hands free set, etc. to a mobile telephone. For these reasons, a multiple mode antenna (that is, an antenna which can resonate at different frequencies to allow a communication device to operate in multiple bands) is highly desirable.
Some dual band antenna designs are known. The Japanese patent (6-37531) discloses a helix which contains an inner parasitic metal rod. In this antenna, the antenna can be tuned to dual resonant frequencies by adjusting the position of the metal rod. Unfortunately, the bandwidth for this design is too narrow for use in cellular communications. Dual band printed monopole antennas are known in which dual resonance is achieve by the addition of a parasitic strip in close proximity to a printed monopole antenna. While such an antenna has enough bandwidth for cellular communication, it requires the addition of a parasitic strip. Moteco AB in Sweden has designed a coil matching dual band whip antenna and coil antenna, in which dual resonance is achieved by adjusting the coil matching component (1/4.lambda. for 900 MHz and 1/2.lambda. for 1800 MHz). While this antenna has relatively good bandwidth and radiation performances, its length is only about 40 mm. A non-uniform helical dual band antenna which is relatively small in size is disclosed in Applicant's copending, commonly assigned U.S. patent application Ser. No. 08/725,507, entitled "Multiple Band Non-Uniform Helical Antennas", the entirety of which is incorporated herein by reference.
Typical dual band extendable whip antennas, such as those mentioned above, require a complicated matching network to match the whip antenna impedance to the two bands within 50 ohms. A dual band retractable antenna is disclosed in Applicant's copending, commonly assigned U.S. patent application Ser. No. 08/725,504, entitled "Retractable Multi-Band Antennas", the entirety of which is incorporated herein by reference. Such an antenna includes two ports, one for a helical antenna and another for a whip antenna and a device for switching therebetween.
It would be desirable for a multiple mode portable communication device to have an efficient multiple band antenna. Conventional dual band helical antennas such as those described above have certain disadvantages. For example, mechanical production tolerances can change the resonant frequencies, particularly at higher bands. Also, it can be relatively difficult to provide a sufficient coupling for dual band parasitic coupling extendable antennas because the distance between a base antenna and the extendable whip can be different at different bands.